[Get Started] Update Grids Documentation to leverage new Responsive Grids

This PR updates the Grids documentation and increases its synergy with the Get Started page (does that make sense?). I've also updated the styles and included a couple more partials and examples. **Notably, the grid helper classes have been removed**. 

If anyone wants to test out this PR, you'll have to test against your local Pure repo. You'll need to have the Pure repo set to the `remove-responsive-grid` branch and then use `bower link` to serve the local Pure copy when running the website.

```
$ bower link
$ cd ../pure-site
$ bower link pure
$ npm start
```

## Screenies

### Responsive Images
![](http://f.cl.ly/items/1O2B083X2T1P3Z0o1L32/Screen%20Shot%202014-02-28%20at%203.59.06%20PM.png)


### Default Media Queries
![](http://f.cl.ly/items/1h3O450J1F0K3O1m1n3n/Screen%20Shot%202014-02-28%20at%203.59.27%20PM.png)